The invention relates to a process for producing organically modified aerogels by preparing a sol comprising [SiO4/2] units and [RxSiO(4−x)/2] units, where x is 1, 2 or 3 and may be the same or different and R is hydrogen or a substituted or unsubstituted organic moiety and may be the same or different, forming a gel out of the sol, surface modifying the resultant gel in the presence of above 0.1 wt % of a compatibilizer in a mixture comprising organosiloxane and initiator, wherein the mixture comprises not less than 20 wt % of organosiloxane, and wherein the initiator consists of an acid or a chlorosilane or mixtures thereof, and drying the gels obtained.
Aerogels are highly porous rigid solids in that their volume is up to more than 95% pores. Whereas a lyogel represents a liquid-filled structure, the pores of an aerogel are air filled. In a hydrogel, which represents a special instance of lyogel, the pore liquid is not less than 50% water. Owing to their porous structure, aerogels have a high specific surface area, low pore diameters and a particularly low density. These properties make aerogels ideal materials for applications in thermal insulation.
While there are various species of aerogels, those based on silicate are the most widely used, being of particular technical relevance because of their low flammability.
Aerogels comprise an arborization of particle chains having very many interspaces in the form of open cells. These chains have contact points, ultimately resulting in the picture of a stable, spongelike network.
The process of preparing an aerogel is in principle very simple. A first step comprises preparing a corresponding lyogel and a second step comprises drying, i.e., exchanging the solvent for air.
The drying step, i.e., the step of removing the pore liquid, is that step of the process which is determinative for the quality of aerogels. Destruction of the gel structure has to be avoided in the course of this step. There are in essence two strategies for this:                1) “Supercritical drying”, i.e., pressure and temperature conditions above the critical point, can be used to ensure that the gel retains its structure and does not shrink or collapse. Capillary forces and hence destruction of the network are substantially avoided in the supercritical domain. The disadvantage of this method is that some technically burdensome, costly high-pressure technology is required for this process and therefore the process is difficult to realize on a large industrial scale, especially as a continuous process.        2) The same result is attainable by drying at atmospheric pressure provided the pore surface was passivated beforehand by modification (silylation). Hence silylation of the free silanol groups in the gel is a way to substantially avoid the gel structure shrinking irreversibly during drying. Since said modification usually utilizes hydrolysis-sensitive chemicals such as trimethylchlorosilane and hexamethyldisilazane, a solvent exchange is generally carried out first. Solvent exchange involves two or more steps wherein the water-containing pore liquid is replaced by inert organic solvents such as hexane in order to avoid the hydrophobing agent (trimethylchlorosilane for example) reacting with the water of the pore liquid.        
In addition to stabilizing the structure in the drying step, surface modification leads to a hydrophobicization of the outside and inside surfaces of aerogels. There are many applications where an adequate hydrophobicity is absolutely essential. Especially the field of building insulation requires insulants to be permanently water-repellent, which is why hydrophobic materials are preferred for such applications.
The high specific surface area of aerogels augurs the use as carrier material and transfer agent in chemistry, for catalysis say, or in medicine. Aerogels are by virtue of their specific surface area further also useful as absorbent or filter materials.
The most striking characteristic of aerogels includes their extraordinarily low thermal conductivity. This high insulating effect is made possible by the special construction of aerogels, especially their extraordinary porous structure (densities below 0.2 g/cm3, mesopore volumes above 3 cm3/g and pore diameters below 25 nm).
Thermal insulation is an important aspect if energy consumption is to be reduced. Especially the field of building insulation is where conventional, inexpensive insulating materials such as polystyrene, polyurethane and glasswool are increasingly coming up against the limits dictated by their high flammability and/or limited insulating effect.
For the use of aerogels to be competitive, an inexpensive method of production is vital. It is accordingly advantageous to minimize the number of processing steps which have to be carried out and, in particular, to preferentially eschew time-consuming operations such as a multi-step solvent exchange.
EP 0 948 395 B1 accordingly disclosed the development of a method for producing organically modified aerogels wherein a hydrogel is surface modified directly, without first exchanging the aqueous pore liquid for organic solvents. The examples utilize a sodium waterglass solution or silicon tetrachloride as SiO2 source and hexamethyldisiloxane (HMDSO, (CH3)3Si—O—Si(CH3)3), trimethylchlorosilane (TMCS, (CH3)3SiCl) or trimethylsilanol ((CH3)3SiOH) for modification. The free OH groups of the hydrogel react therein with the silylating agents to form oxygen-bound trimethylsilyl groups (TMS, (CH3)3SiO1/2). When the silylation is carried out by reacting some of the water in the pores of the hydrogel with the silylation medium used (e.g., TMCS) to form the water-insoluble hexamethyldisiloxane, the volume of the compound formed will necessarily displace at least some of the water out of the pores. This, during the silylation of the inside surface of the network, leads to a concurrent, complete or partial exchange of liquid in the pores of the hydrogel for the water-insoluble medium.
The method disclosed has the disadvantage that the surface modification (silylation) either takes place at high temperatures of 80-100° C. or requires a very long reaction period of several days. Only the use here of large amounts of HCl and/or trimethylchlorosilane will ensure a rapid and complete form of surface modification. During the hydrophobing step, the pore liquid is displaced out of the gel and replaced by HMDSO, in which connection the authors of this patent, F. Schwertfeger and D. Frank, in a subsequent publication with M. Schmidt in the Journal of non-Crystalline Solids (vol. 225, pp. 24-29, 1998), specify that a complete exchange of the pore liquid requires not less than 15 mol % of TMCS based on the pore water, corresponding to 81.5 g of TMCS per 100 g of hydrogel (see sample 2 in table 1), to obtain a complete exchange of the pore liquid and hence aerogels of low density (below 140 kg/m3). And 80 ml of HMDSO are by-produced per 100 g of hydrogel. So a disposal issue is created in addition to costs being incurred for the raw material. The aqueous HCl partly contaminated with salts is generally unrecyclable, but has to be disposed of via the wastewater. The high excess of trimethylchlorosilane generates a large amount of hexamethyldisiloxane, which needs an additional processing step to convert back into trimethylchlorosilane. Similarly, the reaction heat generated by the use of large amounts of TMCS requires an increased engineering effort on the process design side. For that reason, but also in order to minimize the amount of hazardous substances and thereby increase processing safety, the amount of hydrochloric acid and trimethylchlorosilane should be minimized.
The authors of the patents CN 101691227 and CN 102897779 likewise carry out a surface modification of the lyogel in order to be able to do away with supercritical drying and hence reduce manufacturing costs, and also to provide a simple process for producing silicated aerogels. In contradistinction to EP 0 948 395 B1 and the cited publication by Schwertfeger, Frank and Schmidt, CN 101691227 and CN 10897779 utilize silicated gels already “premodified” by cocondensation of methyltrimethoxysilane (MTMS) or methyltriethoxysilane (MTES) with SiO2 sources such as waterglass or TEOS and, following a solvent exchange, surface modify the resultant silicated lyogels in a second step with a solution of TMCS in hexane. However, the solvent exchange and the dilution of the silylating agent in hexane lead to very long reaction times, which compromises implementation on a large industrial scale. The handling of chlorosilanes such as TMCS has the disadvantage that open systems cannot be used at comparatively high temperatures because of the low boiling point of TMCS (57° C.) for example. TMCS is further classified as a flammable, corrosive and toxic substance. Substituting hydrochloric acid for TMCS, as is possible with the use of HMDSO as solvent, is not possible with the use of hexane as solvent. After the reaction, hexane, TMCS and also the HMDSO formed in the course of the reaction of TMCS with the pore water have to be separated off or disposed of, which creates additional costs.